Digital signature of text messages and of other fixed-type data sent across or through networks is common and well established. The digital signatures are intended to simulate the security properties of handwritten signatures on paper and, to that extent, serve to authenticate the documents, data or items being transmitted. In general, using a key generation algorithm, a sender generates a private key for use by the sender and a public key for use by all others including the intended recipient of the message containing the fixed data. Then, using a signature algorithm, the original document is signed to generate a signature which is transmitted together and sometimes embedded within the original document. The recipient uses the public key and the original document to verify the signature and to prove the authenticity of the document as being from the sender with the private key and as being the document signed by the sender.
Digital signature of signals that change over time such as for example the human auditory signal presents problems, particularly when transcoding of the original message is required. As an example, a voice message may be recorded by a sender in a first format such as by using a G.711 standard encoding scheme also known as pulse-code modulation (PCM) wherein signals of voice frequencies are sampled at a selected rate such as about 8000 samples per second. The message may be broadcast onto or through a network and obtained by one or more receivers. However, some of the message recipients may utilize a different decoding scheme at their respective endpoints. As an example, a message may be received at an IP phone using G.722 improved standard encoding scheme. Thus, a digital signature associated with the original message encoded by the sender using a first scheme such as G.711 could not be used to verify the authenticity of the original message by receivers or recipients using a different decoding scheme such as G.722 for example.